Home Estuary Pontoon Offshore Habitat Mapping References

Collection

In order to study the phytoplankton present in the water column, samples were collected from a range of depths off the Falmouth coast. These phytoplankton samples, of 100ml each, were placed in dark glass bottles containing 1ml of Lugols, in order to preserve them.

Preparation

On returning to the laboratories the water samples were placed in a settling column and left overnight, the top 90ml being subsequently removed by a pump. This resulted in a concentration of phytoplankton tenfold greater than would be expected to be observed in the water column. The remaining 10ml had lugols iodine added in order to stain the phytoplankton, before being placed in a sample vial.

Analysis

Each of the fourteen sample vials were divided among group members, one ml of a sample was pipetted into a Sedgewick Rafter, which was placed under a light microscope. At least 50 squares were then counted, with a tally of phytoplankton species counted and each individual identified, to species level where possible. Calculations were then run to work out the total number of individuals per millilitre of estuarine water.

Offshore Biology

Results and Discussion:

In all four stations diatoms dominate the phytoplankton biomass, being the sole phytoplankton present in all but station C25. As the depth increases, so does the relative proportion of dinoflagellates, cryptophtes and ciliates. At forty metres and below, across all stations, cryptophytes become far more prevalent, with the complete exclusion of diatoms at station C27. Station C27 also seems to have the highest proportions of dinoflagellates, the highest numbers being at intermediate depths of 34 metres. As the stations were sampled during spring (July), it is to be expected that diatoms dominate the numbers, due to their high success rates in spring blooms, outcompeting other species. At depth, where photosynthesis is limited by the depth of the euphotic zone (light attenuation decreases exponentially with depth in the water column), the cryptophyte proportions therefore increase, as some cryptophytes can utilise heterotrophic nutrition, rather than relying solely on light.

Collection

Zooplankton samples were collected at four of the five offshore stations by deploying a 200 micron, 50cm diameter closing zooplankton net down to the lower boundary of the deep chlorophyll maximum (DCM), and pulling it up vertically until it reached the upper boundary of the DCM, where it was closed. As the depth that samples were collected from was dependent on the DCM, the sample depth and distance covered varied between stations.

Analysis

Zooplankton samples collected in the field were prepared by inverting the sample bottles, in order to ensure that the collected zooplankton were re-suspended and evenly mixed. A 10ml water sample was extracted from each of the samples collected at each station using a pipette and placed in a measuring cylinder. A third of this sample was then poured into a Bogorov chamber and placed under a light microscope so that the zooplankton could be identified and counted. This process was repeated until the entire 10ml water sample was analysed for each station. Count data per 10ml were then used to calculate a population density value for each zooplankton group per m3 at each station, taking into account the depth range of the seawater sample at each location.

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Phytoplankton

Zooplankton

Offshore Results

At all stations, zooplankton populations were dominated by Copepoda, with this group showing the strongest dominance at station C29 (comprising 86% of the population of groups found in 1m3 of seawater, 88% of the population if Copepoda nauplii are included). Copepoda exhibited its weakest dominance at station C27 (comprising 65% of the population of groups found in 1m3 of seawater, 75% of the population if Copepoda nauplii are included). Siphonophora generally showed the next highest level of population dominance after Copepoda and Copepoda nauplii across all of the stations. It is worth noting that at station C27, where both shallower (40 – 25m) and deeper (50 – 40m) samples were taken, this pattern was not as pronounced, with no group showing clear dominance after Copepoda. The dominance of the Copepoda group across all of the stations is to be expected, as they are highly abundant in marine systems and play an essential role in marine food webs by controlling phytoplankton abundance (Turner, 2004).

Turner. J, (2004); The Importance of Small Planktonic Copepods and Their Roles in Pelagic Marine Food Webs; Zoological Studies, 43: 255-266

Results and Discussion: